The invention relates generally to the art of frequency synthesizing and more particularly to frequency synthesizers utilizing a phase locked loop including a frequency conversion mixer.
It is well known in the art to synthesize frequencies by means of phase-locked loop circuits. The phase-locked loop (PLL) includes a tunable oscillator (typically a voltage-controlled oscillator (VCO)) whose output is locked to a known reference signal by means of a phase comparator. The phase comparator generates an output voltage or current that is proportional to the phase difference between the two signals. The phase comparator output is fed back to the input of the VCO and used to tune the VCO. This forces the VCO output to have exactly the same frequency as the reference signal. By interposing a divide-by-N block in the circuit comparator, the reference frequency may instead be compared with the VCO frequency divided by N; the VCO output will then be locked to N times the reference frequency. By varying N, it is possible to generate frequencies which are the Nth harmonics of the reference frequencies, where N is an integer. Another technique, called fractional N, makes it possible to generate frequencies that are any rational multiple of the reference frequency. Such a technique is disclosed in U.S. Pat. No. 3,928,813 issued to Charles A. Kingsford-Smith on Dec. 23, 1975, entitled "Device for Synthesizing Frequencies which are Rational Multiples of a Fundamental Frequency".
Utilization of a frequency mixing (frequency addition and subtraction) means in a phase-locked loop to synthesize selected output frequencies is also well-known in the art. The use of a series of stepping or reference frequencies with such a PLL will provide a continuous constant increment frequency coverage over a given output frequency (F.sub.out) range. Typically, such frequency synthesizers utilize a series of stepping frequencies (F.sub.step) equal to N times a reference frequency (F.sub.ref) where N is an integer. To avoid frequency discontinuities over a given output range, F.sub.ref must be equal to or less than the difference between the highest and lowest input frequencies (F.sub.in) applied to the PLL. This relationship can be expressed as follows: EQU F.sub.ref .ltoreq.(f.sub.b 31f.sub.a)
where f.sub.b and f.sub.a are the high and low input frequencies, respectively. The range of F.sub.in typically covers an octave, i.e., f.sub.b =2f.sub.a. Also, it is typically desired that F.sub.out cover an octave, i.e., f.sub.2 =2f.sub.1, where f.sub.2 and f.sub.1 are the high and low output frequencies, respectively. The goal of such a PLL is generally to translate the characteristics of a range of input frequencies to higher frequencies in the range of the reference frequency. The condition f.sub.b =2f.sub.a is necessary for complete phase-locked coverage over the range of F.sub.out in loops of the type discussed herein.
One technique for the generation of the stepping frequencies utilizes a harmonic frequency generator in combination with selective filters. However, the minimum necessary frequency separation between adjacent stepping frequencies to provide continuous constant increment frequency coverage over the synthesizer output frequency range makes it difficult to filter out unwanted frequencies resulting in the injection of spurious signals into the PLL. Furthermore, the generation of stepping frequencies becomes significantly more difficult and expensive as the frequency of the desired synthesizer output increases. Frequency division is the only technique other than frequency mixing to generate F.sub.out at a higher frequency than F.sub.in ; however, frequency division produces noise in F.sub.out which is the divide ratio times any noise in F.sub.in.
U.S. Pat. No. 3,902,132 issued to Raymond L. Fried on Aug. 26, 1975, entitled "Closed Loop Variable Frequency Signal Generator", discloses a frequency synthesizer comprising a phase-locked loop utilizing a frequency mixing circuit and a series of stepping frequencies to provide a selected range of output frequencies. Fried discloses a loop that can both add and subtract F.sub.in to and from F.sub.ref, hence requiring fewer reference frequencies than prior art synthesizers which synthesized F.sub.out by either adding or subtracting F.sub.in to or from F.sub.ref. Further, the required reference frequencies need be separated by 2f.sub.a rather than f.sub.a as utilized in prior art synthesizers. (Where f.sub.a and f.sub.b are the low and high frequencies respectively of F.sub.in and f.sub.b =2f.sub.a.) F.sub.ref (F.sub.step) is a combination of frequencies represented by the equation: ##EQU1## The appropriate signal is selected by a tunable filter; the wider the spacing of the reference signals, the easier it is to filter out unwanted signals thereby reducing spurious noise at the synthesizer output. The technique disclosed by Fried utilizes four reference (stepping) frequencies to cover one octave at the synthesizer output.